According to "the weight of coal pile A is two fifths of the weight of ethylene and propylene", we can know that A accounts for 2/7 of the total, and according to "the weight of coal pile B is a quarter of the weight of A and C", we can know that B accounts for 1/5 of the total, so we can know that C accounts for 1-2/7- 1 of the total.

B is 175* 1/5=35 tons.